In recent years, microwave discharge light source having an electrodeless bulb disposed in a microwave resonance cavity has been developed and is attracting attention because of its long life. FIG. 1a shows one of such microwave discharge light source apparatus disclosed in Japanese Laid-Open Patent Application 56-126250; FIG. 1b shows a modification thereof disclosed in Japanese Laid-Open Patent Application 57,-55091. In both apparatuses, a magnetron 1 having an antenna 1a is disposed at the end of a waveguide 2 having ventilating holes 2a which supplies the microwave generated by the magnetron 1 to a resonance cavity 3 through a microwave supply port 3a; the cavity 3 is formed by a paraboloidal wall 3b having a light reflecting rotationally symmetric inner surface and a metallic mesh 3c forming the front face of the cavity 3, which opaque to microwave but transparent to light. A spherical electrodeless discharge bulb 4 disposed in the cavity 3 and having encapsulated therein a plasma generating medium emits light through the metallic mesh 3c covering the front face of the cavity 3, when the microwave is radiated into the bulb 4: at first, the gas enclosed in the bulb 4 undergoes discharge, due to the microwave radiated into the cavity 3; thus, the inner surface of the bulb 4 is heated, and the metal, such as mercury, deposited on the inner surface of the bulb 4 is evaporated into a gas; as a result, the discharge in the bulb 4 goes over to that of the metallic gas, in which light having an emission spectrum peculiar to the kind of the metal is emitted from the discharging metallic gas. The emitted light is reflected by the cavity wall 3b and is radiated forward through the front mesh 3c. The apparatuses further comprise a fan 5 at the end wall of the housing 6 for cooling the magnetron 1 and the bulb 4.
Microwave discharge light source apparatuses similar to those described above are also disclosed in U.S. Pat. Nos. 4,498,029 and 4,673,846, both issued to Yoshizawa et al. The first of these U.S. Patents teach an apparatus in which the bulb is sufficiently small to act substantially as a point light source; the second teach an apparatus in which the wall surface of the microwave resonance cavity having the electrodeless bulb disposed therein is mostly constituted by a mesh, wherein the wires constituting the mesh are electrically connected each other without any contact resistance.
A conventional power supply circuit for a magnetron is disclosed in Japanese Laid-Open Utility Model Application 56-162899, or in the first of the above mentioned U.S. Patents, according to which a commercial voltage source at 50 to 60 Hz is coupled to a step-up transformer, and the resulting stepped-up high-voltage AC current is rectified by a full-wave rectifier circuit to obtain pulsing unidirectional current which is supplied to the magnetron. As the rectification is effected by a full-wave rectifier circuit, the resulting high voltage rectified current pulsates at 100 to 120 Hz; consequently, the magnetron generates a microwave pulsing at 100 to 120 Hz. Thus, when magnetron 1 is supplied by this conventional circuit, the discharge in the bulb 4 is caused by the microwave pulsing at 100 to 120 Hz.
The disadvantage of this type of conventional power supply circuit is as follows. First, as the commercial AC voltage of relatively low frequency, i.e., 50 to 60 Hz, is directly supplied to the primary winding of the step-up transformer to obtain a high voltage needed to supply the magnetron, the transformer should be provided with a heavy iron core; the weight of the transformer is equal to or greater than 10 kg when the input power to the magnetron is 1.5 kW. Second, as a full-wave rectifier circuit is used to rectify the AC current induced in the secondary winding of the transformer, neither one of the terminals of the secondary winding can be grounded; thus, the over-all size of the transformer should be further increased to ensure an electrical insulation thereof; in addition, extremely high voltage may develop in portions within or outside of the transformer, which diminishes the reliability of the parts thereof. If the rectifier circuit coupled to the secondary winding of the transformer is constituted by a half wave rectifier circuit, one terminal of the secondary winding of the step-up transformer can be grounded to minimize the above-mentioned drawbacks of the conventional power supply circuit. This, however, causes another problem: as the voltage applied to the magnetron 1 is reduced to 0 during the half period of the commercial AC voltage cycle, the generation of the microwave is stopped for about 8 to 10 ms; thus there is the danger that the discharge is extinguished during the same time intervals. Thus, a full-wave rectifier circuit must have been used to rectify the outputs of the step-up transformer.
FIG. 2a shows an inverter type power supply circuit for a magnetron taught in Japanese Patent Publication 60-189889, wherein the magnetron 1 is supplied by the circuit as described in what follows. A rectifier circuit 8 is coupled across the lines of a commercial AC voltage source E; a pair of series-connected capacitors Cl and C2 are coupled across the output terminals of the rectifier circuit 8 to obtain a substantially constant voltage DC power. An oscillator circuit 9, which comprises a Zener diode Zn, a capacitor C3, a plurality of resistors, and an amplifier A, is coupled across the capacitor C2 to output a rectangular waveform signal having a frequency substantially higher than that of the commercial AC voltage source E to a control circuit 10 comprising a transistor T1, a diode D1, and a plurality of resistors; the frequency of the rectangular waveform signal of the oscillator circuit 9 is determined by the values of the resistors and the capacitor C3 thereof. The control circuit 10 controls the alternate switching actions of a switching circuit comprising the power transistors 11 and 12 and the controlling transistors 11a and 12 a therefor. Namely, by alternately turning on and off the controlling transistors 11a and 12a, the circuit 10 alternately turns on and off the power transistors 11 and 12 in response to the output signal of the oscillator circuit 9. Thus, a high frequency rectangular waveform AC current is supplied to the primary winding P of the transformer T through a filter circuit 13. The AC voltage induced in the secondary winding S of the transformer T is rectified by a voltage doubler rectifier circuit consisting of a capacitor C4 and a diode D2, and is supplied therefrom to the magnetron 1.
The inverter type power supply for a magnetron as described above also suffers disadvantages. Namely, as the magnetron 1 constitutes a non-linear load, the output power and current thereof and the inverter current supplied to the step-up transformer become unstable when the voltage level of the voltage source E fluctuates; the over-current resulting therefrom may destroy the power transistors 11 and 12.
FIG. 2b shows another inverter type power supply circuit for a magnetron taught in Japanese Laid-Open Patent Application 62-113395, wherein the magnetron 1 is supplied by the circuit as follows. A diode bridge rectifier circuit 8 comprising four diodes Do is coupled across the commercial AC voltage source E; a smoothing filter circuit 9 consisting of a capacitor Co is coupled across the output terminals of the rectifier circuit 8 to output a substantially constant DC voltage therefrom. The switching circuit 10 comprises switching transistors Q1 and Q2 and diodes D1 and D2 for reverse currents coupled across the source and the drain thereof, respectively, the transistors Q1 and Q2 being coupled across the negative output terminal of the filter circuit 9 and the terminals P1 and P2 of the primary winding P of the transformer T, respectively. The positive output terminal of the filter circuit 9 is coupled to the center tap 0 of the primary winding P f the transformer T. The gate terminals g1 and g2 of the transistors Q1 and Q2, respectively, is coupled to the center tap 0 of the primary winding P of the transformer T. The gate terminals g1 and g2 of the transistors Q1 and Q2, respectively, are coupled to the output terminals of a control circuit 11. The voltage doubler rectifier circuit 12 consisting of series-connected capacitor Cl and a diode D3 is coupled across the terminals S1 and S2 of the secondary winding S of the transformer T; the negative output terminal d of the rectifier circuit 12 is coupled to the cathode K of the magnetron 1, which is heated by a filament current supplied thereto from a commercial AC voltage source through an electrically insulating transformer (not shown) and the lines h; the positive output terminal f of the rectifier circuit 12, on the other hand, is coupled to the anode A of the magnetron 1 through a resistor R, the terminals of the resistor R being coupled to the input terminals of the control circuit 11.
The control circuit 11 outputs pulses to the transistors Q1 and Q2 at a varying frequency centered around a fixed frequency, to alternately turn on and off the transistors Q1 and Q2. Thus, the current flows alternately from the center tap 0 to the terminal P1 and to the terminal P2 of the primary winding P of the transformer T to induce an AC voltage in the secondary winding S thereof, which is rectified by the rectifier circuit 12 and supplied therefrom to the magnetron 1. The pulse signals of the control circuit 11 at the fixed frequency are subjected to frequency modulation utilizing a modulating signal having a frequency which is lower than the frequency of the fixed frequency of the output pulse signals, to prevent flickering of the discharge in an electrodeless bulb such as those shown in FIGS. 1a and 1b; the flickering of the discharge is caused by an acoustic resonance in the bulb due to the ripple or fluctuation of the microwave energy. Further, the circuit 11 varies the length of time during which the transistors Q1 and Q2 are turned on, so that the output power of the magnetron is held constant irrespective of the fluctuation in the voltage source level; this can be effected by detecting the magnetron current by means of the voltage drop across the resistor R, thanks to the substantially constant voltage characteristic of the magnetron 1.
The inverter type power supply circuit for a magnetron described just above is small-sized and is effective to a certain degree to prevent the flickering of the discharge arc of the electrodeless discharge bulb, thanks to the adoption of the high frequency inverter in the circuit The flickering of the discharge arc, however, may persist even in the apparatuses supplied by the circuit, depending on the kind and amount of the material encapsulated in the bulb and on the microwave energy level radiated into the bulb: the flickering of the arc is particularly manifest when a metal halide compound such as sodium iodide is encapsulated in the bulb in addition to mercury and a starter rare gas, or when the microwave energy supplied to the bulb is at a high level. Further disadvantage of the circuit of FIG. 2b is that the controlling circuit 11 thereof has a complicated structure, because the pulse signals thereof are subjected to frequency modulation and the length of the turning-on time of the switching is varied to maintain the output power of the magnetron 1 at a constant level.
Power supply circuits for a magnetron utilizing inverters are also disclosed in U.S. Pat. No. 4,593,167 issued to Nilssen and U.S. Pat. No. 3,973,165 issued to Hester. The first of these U.S. patents teach a power supply circuit for a magnetron of a microwave oven including an inverter, wherein the step-up transformer exhibits relatively high leakage between its input and output windings and a capacitor is connected across the step-up transformer's output winding; further, a rectifier and filter means is connected in parallel with the capacitor, and supplies substantially constant DC voltage to the magnetron. The second U.S. patent teach an inclusion of an inverter in a power supply for a magnetron which supplies microwave energy to a microwave oven, etc, wherein the DC current obtained by rectifying a commercial AC voltage of 60 Hz is supplied to the step-up transformer through an inductor, which prevents high frequency currents or voltages to flow into the AC voltage source lines. Further, Japanese Laid-Open Patent Application 62-290098 teaches a microwave discharge light source apparatus including an inverter type power supply circuit for the magnetron, wherein the inverter frequency is set at a few tens kHz, for example, thereby maintaining parameters of the plasma in the bulb at a substantially constant level to prevent the flickering of the discharge in the bulb.